1. Field of the Invention
The present general inventive concept relates to an illumination unit to emit light and a projection type image display apparatus employing the same, and more particularly, to an illumination unit with an improved prism, which synthesizes light emitted from a compact light source, and a projection type image display apparatus employing the illumination unit.
2. Description of the Related Art
In general, illumination units include a light source emitting light in one direction and an illumination optical system transmitting the light emitted from the light source. The illumination units are widely used in projection type image display apparatuses that create an image using an image-forming device (e.g., a liquid crystal display (LCD) device or a digital micromirror device (DMD)) that cannot emit light.
In recent years, illumination units and projection type image display apparatuses using a small light-emitting device, such as a light-emitting diode or a laser diode as a light source, have been developed.
Since small light-emitting devices can emit beams of red, blue, and green wavelengths, single-panel projection type color image display apparatuses employing the small light-emitting devices do not need an additional color wheel for obtaining a color image. However, to emit various color beams, a plurality of small light-emitting devices and a structure for synthesizing the various color beams are required.
Referring to FIG. 1, a conventional illumination unit includes first, second, and third LED light sources 11, 12, and 13 disposed at different positions and respectively emitting beams of red, blue, and green wavelengths, and a trichroic prism 20 propagating the beams emitted from the first, second, and third LED light sources 11, 12, and 13 along the same path.
The trichroic prism 20 includes first, second, and third prisms P1, P2, and P3, a first color filter 21 disposed between the first prism P1 and the third prism P3, and a second color filter 25 disposed between the second prism P2 and the third prism P3. The first and second color filters 21 and 25 selectively transmit or reflect incident light according to a wavelength of the incident light. For example, the first color filter 21 reflects a first beam R of a red wavelength and transmits a second beam G of a green wavelength and a third beam B of a blue wavelength. The second color filter 25 reflects the third beam B and transmits the first and second beams R and G.
Accordingly, the first beam R incident on an exit surface 20a of the first prism P1 from the first LED light source 11 is totally reflected based on the principle of critical angle total reflection to be directed toward the first color filter 21. The first beam R is reflected by the first color filter 21, and then transmitted through the exit surface 20a of the first prism P1. The second beam G is sequentially transmitted through the second and first color filters 25 and 21, and propagates along the same path as the first beam R. The third beam B is totally reflected, based on the principle of critical angle total reflection, on a surface 20b of the third prism P3 facing the first prism P1 to be directed toward the second color filter 25, reflected by the second color filter 25, transmitted through the first and third prisms P1 and P3, and then propagates along the same path as the first and second beams R and G. Hence, the first, second, and third beams R, G, and B respectively emitted from the first, second, and third LED light sources 11, 12, and 13, which are disposed at different positions, are synthesized to propagate along the same path.
Meanwhile, for the purpose of totally reflecting the third beam B on the surface 20b of the third prism P3, the first prism P1 and the third prism P3 are spaced a predetermined distance from each other to form an air gap Gair therebetween. That is, to cause critical angle total reflection, there must exist a refractive index difference between the third prism P3 and a surrounding medium, as well as an angle between the surface 20b of the third prism P3 and the third beam B.
Accordingly, when the beams emitted from the plurality of light sources are synthesized using the trichroic prism 20, an optical arrangement of the trichroic prism 20 is difficult to establish.
Further, when the beams are synthesized using the trichroic prism 20, because of the optical arrangement of the trichroic prism 20, it is difficult to employ additional light sources for emitting beams other than the red, blue and green beams, for example, yellow, magenta, and cyan beams. Accordingly, an allowable color gamut is limited in the conventional illumination unit.
Hence, since a projection type image display apparatus employing the conventional illumination unit creates an image by combining the three color beams, it is difficult to use light sources to emit four or more color beams due to limitations of the conventional illumination unit.